The input range of an A/D converter must be designed in such a way that the peak input signal is reliably covered. It means that when (1) the signal amplitude fluctuates in a very large dynamic range, (2) the quantization accuracy must be kept for the smallest signal and (3) the signal needs to be kept linear, a very high resolution A/D converter has to be used. In maw cases, high resolution is merely designed for covering the signal dynamic range rather than the quantization accuracy. For example, in order to obtain a 60 dB signal dynamic range and a minimum 6-bit quantization accuracy, the resolution must be at least 16-bit, a very high demand at high speed. Actually, in such a converter, the resolution for large signals are unnecessarily high. It would be more rational, if the converter offers the same resolution for large end small signals within this range.
Moreover, the trend of low power and low voltage reduces the actual input range, which makes the design of a wide dynamic range A/D converter more difficult since non-ideal factors like component mismatch and amplifier offset are not reduced with the reduction of supply voltage. In such a case, it is very difficult to satisfy large dynamic range with high resolution.
Traditionally, a logarithmic amplifier is used for compressing the signal amplitude in order to expand dynamic range. The accuracy, however, will be seriously degraded for a large compression ratio due to difficulties in designing the logarithmic amplifier. In order to produce linear digital output code a look-up table is usually used, which has to be matched with the amplifier precisely.
The demand on very high resolution A/D converter can be eliminated by the invented floating-point A/D converter, when the resolution is merely needed for covering signal dynamic range. Unlike the known logarithmic amplifier solution, the floating-point A/D converter gives a linear digital output directly. For large and small signals, the effective resolutions are kept constant (or quasi-constant to be accurate), similar to a floating-point number representation. Its resolution and dynamic range can be designed independently, which makes this invention very useful and flexible.